Justification:
Listed as Least Concern because it is extremely widespread, although there are some threats to the species habitat, it does occur in several protected areas and its population is not declining fast enough to qualify for listing in a threatened category.

This species occurs in boreal and montane regions of Labrador, Nova Scotia, and New England across Canada to east-central Alaska (Cook et al. 1997), south to the northern Great Lakes region and in the western mountains to mid-California, Nevada, Utah, and New Mexico in the United States. A small, disjunct population in Arizona's White Mountains has not been observed in recent years (Hoffmeister 1986); there is another apparently disjunct series of populations in the Appalachians ranges from southwestern Pennsylvania to North Carolina, Tennessee, and Georgia.

Pagels (1990, 1991) noted that in the southern Appalachians the water shrew is limited to prime habitat: "high elevation situations where moist, cool, shaded situations have prevailed throughout historic time…elevations ranging from about 760 m (2,500 ft.) in Pennsylvania to 1,150 m (3,800 ft.) and above in North Carolina and Tennessee."

This species has a large range in the boreal and montane regions of North America, and its population is secure, if not abundant, throughout the northern part of the range; in the south, its habitat has been fragmented since the retreat of the last glaciers, making isolated populations vulnerable to extirpation; some subspecies are rare enough to be of concern.

Information on population density is limited. Population densities in the Appalachians appear to be quite low, though numbers are not available (Pagels et al. 1991). In Canada, Banfield (1974) reported that this shrew is usually uncommon but sizeable populations sometimes occur in favourable locations. Generally numbers are highest in summer when young are born and drop off sharply in autumn, with very little decrease through winter.

This species is most abundant along small cold streams with thick overhanging riparian growth. Also around lakes, ponds, marshes, bogs, and other lentic habitats. Though normally associated with water, water shrews "have been found more than 100 m (328 ft) from streams in mature northern hardwood stands in northern New Hampshire" (DeGraaf and Rudis 1986). This and other captures far from water probably represent dispersing individuals.

Nest sites are near water in underground burrows, rafted logs, beaver lodges, and other areas providing shelter. It breeds from February to August in Montana (Conaway 1952). Gestation lasts probably 3 weeks. Litter size is 3-10, average 6; with 2-3 litters/year (Montana). Optimum conditions in captivity, or abundant food supply in nature, may increase the number of litters (Churchfield 1992). They sexually mature in their second calendar year. Their maximum lifespan is about 18 months (Beneski and Stinson 1987).

Common predators include fishes such as trout, bass, and pickerel, minks, otters, weasels, snakes, and occasionally, hawks and owls (Merritt 1987). The water shrew is primarily dependent upon aquatic insects; but also eats various other invertebrates. It may take small vertebrates (fishes, amphibians) when available. It hunts under and on top of water, and may even be seen running across the water surface. Water shrews, with their high metabolic rates, need to consume approximately their weight in food every day (Conaway 1952, Sorenson 1962). In the wild they seem unable to store significant body fat and can die of starvation within a few hours. When a surplus of food is available, it is often horded, the shrew sometimes defecating on it to keep other shrews away.

Churchfield (1992) stated that water shrews (both S. palustris and Neomys sp.) "are particularly vulnerable to the destruction of their aquatic habitats through pollution and drainage." Activities such as logging, agriculture, road building, and surface mining contribute to the loss of habitat. Logging may not be a serious threat if water quality is protected and buffer strips are maintained at the site (Christian pers. comm., 1994).

Regarding the subspecies S. p. punctulatus in the east, Pagels et al. (1991) stated: "Perhaps the greatest continuing threats to the water shrew are the loss of additional habitat resulting from infestation of introduced insect species, such as the gypsy moth, or the use of insecticides to control such infestations. The effects of acid rain, particularly on the shrew's microhabitat and food supply could be particularly devastating".Shrews feeding on invertebrates accumulate and concentrate pesticides and heavy metals in their tissues. Northern short-tailed shrews (Blarina brevicauda) were found to contain DDT nine years after application of the pesticide to a forest habitat (Dimond and Sherburne 1969). "Contamination of habitats, and the resultant effects on the food chain, by mining and refining of metalliferous ores, by motor exhausts and even by the application of sewage sludge to land, is becoming more widespread. Shrews from contaminated grasslands have been found to accumulate some of the highest concentrations of metals recorded in wildlife" (Churchfield 1992).

Southern populations are limited to the mountains in both the east and west and consequently tend to be isolated. When an isolated population disappears for any reason, it is unlikely to be restored by natural dispersal. Hence these isolated populations may be especially vulnerable to extirpation from human activities or long-term climate change.

The water shrew is a boreal species, also inhabiting relict habitat in southern mountains. It requires high quality water, preferably mountain streams, and abundant cover such as rocks, logs, or overhanging stream bank. Suitable management consists primarily of maintaining these conditions. There are many areas (e.g., state and national parks) in which habitat is adequately protected. The southern range of the water shrew may be shrinking through habitat fragmentation and local extirpation, but current information is inadequate to allow an evaluation of this hypothesis.